Since it's a little troublesome to hunt bosses for Note Sheets? (forgot how they're called ;D) - It would be nice if there would be a trader, who would sell them too.
If it's too easy, make them only exchange it for a set amount of basic drops or limit them per hour (which would boost up the PvP in certain areas as well, if you place them in PvP Zones.)
Or maybe, hide them across the maps and give them random time windows to appear. - Just an idea of mine.